In certain industries it is valuable to characterise particular materials. To do so, sensors can be configured to move with respect to a particular material and signals passed into or through the material. In some cases, the sensors may be moved while the material remains stationary, while in other cases the material may be moved and the sensors remain stationary. In some cases, the sensors may be mechanically touching the material, in other cases, they may not (e.g. acoustic, optical sensors, or the like).
The data output of such sensors is usable to provide a characteristic of the material being measured. This measured characteristic may provide a characteristic image or map of the material itself, or may be useable to reconstruct further data or information encoded with or within the material. The measured characteristic may be representative of the body of the material, and/or the surface or surface region of the material.
Examples of such moving sensors include a tape head usable with magnetic tape, whereby the tape is moved with respect to a stationary, or relatively stationary, tape head, such that the characteristic magnetism of the tape can be measured. This characteristic can then be used to reconstruct audio, video or data. A further example is an image scanner, or facsimile machine. Again the material (i.e. paper, or the like) is moved relative to an optical sensor. The data output of the sensor is usable to provide an image of the paper, or indeed, what is composed on the paper.
By maintaining the relative speed of such sensors to be constant or at least roughly constant, samples can be taken at particular intervals, such as regular intervals, along the material. These samples can then be used to provide a characteristic of the material.
However, because such sensors are being moved (and/or the material is being moved) certain variations in relative speed can arise. In the examples described above, motors used to wind or pull magnetic tape may have an eccentric motion causing a resultant variation in acoustic frequency (i.e. flutter, etc.) when the signal is reconstructed into an audio characteristic. Similarly, the tape itself may have been pulled or stretched, causing an apparent change in speed having the same effect on a reconstructed audio characteristic. In the case of a scanner, paper may jam, or be pulled awkwardly through a machine, resulting in the commonly-seen elongating or contracting of images when data is reconstructed into a copy of images provided on paper.
To compensate for these variations in relative speed, sensors may be used that mechanically interact with the material being characterised. As a result, variations in speed may be caused due to the sensor temporarily sticking or jamming on the material (e.g. as a result of frictional effects). The release of this sticking or jamming may result in a “ping back” effect, whereby the sensor or material accelerates for a time in the other, or opposite, direction. Other effects, such as the variations in roughness or friction of a surface, etc. may also cause variations in speed.
As a result of sensors experiencing these speed variations (e.g. either because the movement of material/sensor cannot be sufficiently controlled or because of sticking/jamming), it can be difficult to provide accurate data derived from such sensors.
An example of an industry that attempts to provide characteristic maps of a material using relatively moving sensors is the oil and gas industry. One such process that uses such techniques is referred to as logging, where boreholes drilled into the ground are characterised by pulling a measurement module, or so-called logging tool, through those holes. In such cases, it can be valuable to measure precisely the characteristics, or formation, of subterranean material through which a borehole passes. This information might be useful when exploring for oil and gas. For example, it can be useful to determine precisely the formation and location of particular strata formations from pilot boreholes: those used to determine the possible location of one or more hydrocarbon reservoirs. By identifying accurately the locations and presence of particular strata, the location of reservoirs can be suggested.
Such logging tools generally comprise a plurality of sensors. Some of these sensors may, in some cases, be displaced in the direction of travel from each other (e.g. the sensors may be configured in rows). As these logging tools are pulled through a borehole, the sensors are configured to touch, or at least communicate with, the wall of the borehole and characterise the associated subterranean material. However, regardless of how carefully the logging tools are pulled, variations in the speed of the tools occur. In addition, each sensor can experience localised variation due to frictional or so-called rumble effects, or the like.
Accelerometers comprised with sensors/tools have been used in order to correct data derived from tools experiencing a deviation in speed that occurred as the logging tool was being drawn through a borehole. However, there are several problems associated with using such accelerometers, which means that the quality of the results is severely limited. Firstly, due to the high forces involved, two or more different accelerometers are needed for each sensor or in order to cover the range of motion (i.e. high and low acceleration). The nature of the mathematical transformations involved when using accelerometers is approximate. Furthermore, the accelerometers can be difficult to calibrate, and in addition, are rarely calibrated when used in real life. Also, due to the harsh environments, the devices are often damaged due to being dropped or knocked, etc.
Correction for speed deviations of sensors/tools based on accelerometer data, or so-called kinematic correction, is not entirely satisfactory. As the resolving power of sensors, such as those provided with logging tools, increases the deficiencies of existing kinematic correction techniques are becoming more noticeable, and thus the information which can be inferred from such data is limited.
This background serves to set a scene to allow a skilled reader to better appreciate the following description. Therefore, none of the above discussion should necessarily be taken as an acknowledgement that that discussion is part of the state of the art or is common general knowledge. One or more aspects/embodiments of the invention may or may not address one or more of the background issues.